This invention relates to a continuous process for the polymerization of .alpha.-olefins with a metallocene catalyst system using a dilute .alpha.-olefin feed.
Olefin polymerizations, particularly, ethylene/.alpha.-olefin copolymerizations can be broadly differentiated as occurring in solution, suspension, or in the gas phase. Within the continuous solution polymerization category, operating conditions can vary quite broadly depending on such variables as the concentration of the reactants in the total feed, the nature of the catalyst system employed, the desired molecular weight of the polymer, and the desired monomer/comonomer ratio within the final polymer.
When concentrated ethylene and .alpha.-olefins, e.g., propylene, feeds are copolymerized with conventional Ziegler-Natta catalysts, it is known as described in U.S. Pat. Nos. 3,912,698 and 3,637,616 to conduct such polymerizations continuously to obtain an ethylene copolymer, dissolved in the solvent, which is continuously removed and isolated by known means. Unreacted monomers leaving the reactor are recovered and recycled to the reactor along with fresh monomers to replace those polymerized.
It is also known, as in EPA 270,339, to conduct continuous ethylene/.alpha.-olefin copolymerization under highly dilute conditions at atmospheric pressure using conventional Ziegler-Natta catalysts. These processes suffer the drawback that the catalysts exhibit low productivities and produce polymer product exhibiting large molecular weight distributions, high ash content, and number average molecular weights too large to be useful as lubricant additives. Consequently, if low molecular weight polymers are desired either hydrogen must be used to keep the molecular weight of the product low, e.g. less than about 15,000 or the catalyst concentration has to be increased to extremely high levels to obtain low molecular weights. The hydrogen treatment at least partially saturates the terminal double bonds in the product, thereby significantly reducing or destroying the polymer's utility for most functionalization reactions, e.g., those used in the production of dispersants. Saturated polymers have limited applicability for use in subsequent functionalization techniques (e.g., by "ene" reaction with maleic anhydride) which rely on a high terminal double bond content to achieve functionalization.
In contrast, recent developments in the catalyst and .alpha.-olefin copolymer art disclose that metallocene catalyst systems yield low molecular weight polymers with high terminal ethenylidine (vinylidene) content directly, without hydrogenation, as well as other advantageous properties (see, EP Publication Nos. 129,368; 440,504; 440,505; 440,506; 440,507; 440,508; 441,548; PCT Publication Nos. WO 91/11488; WO 90/01503; and U.S. Pat. Nos. 5,017,299; 5,128,056; 5,151,204; 4,704,491; 4,668,834; 4,888,393; and 4,542,199).
More specifically, it is known in the art to employ high pressure/high temperature systems, as in U.S. Pat. No. 5,084,534 and EP Publication 260,999, which utilize pure or nearly pure feeds and metallocene catalyst at pressures up to 2,500 bar and temperatures up to 300.degree. C. Such systems are designed to produce high molecular weight polymers at high catalyst productivities (i.e. grams of polymer produced per gram of catalyst used). These systems suffer a number of drawbacks when applied to low molecular weight polymer production; most notably, expensive pure feeds and specialized equipment resulting in high fixed costs of production.
Also, such systems operate with a single phase to allow efficient mixing of the reactants and, therefore, homogeneity of the product.
A single phase system is achieved by operating at temperatures and pressures sufficiently high to compress the ethylene and make it dense enough to dissolve the polymer product therein. This produces a homogeneous phase of polymer in reactant. To achieve high temperature and reduce the size of the reaction zone the process is run adiabatically (heat is not removed), making temperature control difficult. Since the molecular weight of the product is directly related to temperature, failure to maintain constant temperature throughout the reaction process results in increased polydispersity M.sub.w /M.sub.n (or Molecular Weight Distribution, MWD). Temperature control becomes increasingly more difficult at higher conversions in an adiabatic system. Consequently, conversions in the high temperature/high pressure process are kept to a minimum, about 10%. For polymers having molecular weight of 100,000 or more, variations of +1,000 or so have little effect on MWD. For polymers on the order of 10,000 number average molecular weight, M.sub.n and below, however, such variations are extremely disadvantageous.
Moreover, the use of pure feeds is another limiting factor on the rate of conversion. As the conversion rate in a pure feed system is increased, the concentration of polymer in the reactor increases until it becomes extremely difficult or impossible to mix and pump the reactants efficiently. This problem is exacerbated at a low reaction temperature where the polymer viscosity increases further. The limitations on conversion induced by pure feeds applies to essentially all polymerization processes.
Others have attempted to prepare low molecular weight ethylene .alpha.-olefin copolymers (EAO) at low temperatures and pressures, with metallocene catalyst as described in U.S. Pat. No. 4,704,491 to Mitsui Petrochemical Industries and U.S. Pat. No. 4,668,834 to Uniroyal. The process described in the Mitsui '491 patent operates with high catalyst concentrations, e.g., 10-2 moles/liter, pure undiluted vaporized feeds, at atmospheric pressure, extremely short reactant residence time (e.g., about 0.5 hours), with no recycle of unreacted reactants. The high catalyst concentrations are needed because the mass transfer of the reactants into solution is poor and, consequently, low concentrations of reactants appear in solution. Low conversions are the result. The Uniroyal '834 patent operates at super atmospheric pressure with a compressor driven cooling system and pure undiluted feeds.
Methods employing dilute reaction mixtures and utilizing batch processes are known in the art. Typically, dilution of the reaction mixture occurs as a result of employing a metallocene catalyst system in a diluent, usually toluene. However, the use of a dilute feed of .alpha.-olefin is not found in this art. Moreover, rapid introduction of reactants into solution is often accomplished by introducing the pure reactants directly into the vapor space of the reactor instead of the liquid phase, or by bubbling the reactants up through the reaction mixture at pressures too low to provide effective dissolution therein. Such processes are also conducted at very low monomer conversions.
KAMINSKY, et al., U.S. Pat. No. 4,542,199, describes a batch process wherein pure ethylene and an .alpha.-olefin are introduced into a pressure vessel containing a metallocene catalyst system dissolved in toluene.
LUKER, U.S. Pat. No. 5,023,388 refers to a batch process, wherein the metallocene catalyst system is dissolved in diesel oil in the presence of large quantities of .alpha.-olefin and ethylene and hydrogen gas at 7 bar. The molecular weight distribution of the product is reported to be 2.8.
SLAUGH, et al., EP 366,212 published May 2, 1990, teaches continuous or batch processes, though the examples offered are all batch. The feeds used are pure and the reaction mixture is highly concentrated. The process produces polymer wherein 80 percent of the product has less than 20 carbon atoms per molecule.
TSUTSUI, et al., EP 447,035 published Sep. 18, 1991, refers to a series of batch processes, wherein ethylene is first polymerized or copolymerized with .alpha.-olefin in a first batch under concentrated or dilute conditions; the product is isolated; and then the product is introduced into a subsequent batch process with ethylene or an .alpha.-olefin. The process may be continued to a third round of batch processing. Reactants may be relatively concentrated in one batch, yet relatively dilute in the next or vice-versa.
Another approach, as described by HIROSE, et al., JP 2-173,110 disclosed Jul. 4, 1990, is to recycle massive amounts of ethylene and propylene gas through a solvent-containing reaction vessel. The feeds are pure and the quantity of reactants to solvent is very high. The ratio of ethylene to .alpha.-olefin is necessarily very low in order to prevent polyethylene formation. Polymers formed by this process have ethylene contents less than 10 percent by mole.
It is also known in the art to cool polymerization reactors by evaporation and removal of unreacted monomers from the vapor space, these monomers, being optionally cooled, and recycled to the reactor. Reactors cooled in this manner are referred to as evaporatively cooled reactors or boiling reactors. Polymer is recovered from the reaction mixture by withdrawing polymer solution from the reactor and separating unreacted monomers which are usually recycled to the reactor. Also, as a general proposition, as the concentration of the polymer in solution increases, and/or the molecular weight of the polymer increases, the viscosity of the reaction mixture increases. This in turn reduces the mass transfer of monomer from the gas into the liquid phase and reduces the heat transfer properties of the reaction mixture thereby making it more difficult to cool the reaction mixture. As indicated above, failure to maintain a stable reaction temperature leads to fluctuations in the molecular weight of the polymer and a broadening of the molecular weight distribution.
While evaporative cooling reactors improve heat transfer by removing the exothermic heat of reaction, and can maintain stable reaction temperatures, they have the disadvantage that monomer concentration in solution in the reactor is usually less than its equilibrium value. Thus, as a general proposition, in order to produce a copolymer containing a particular proportion of monomer in evaporatively cooled reactors, it is usually necessary to recycle a larger amount of monomer in the reactor off-gas (to obtain the cooling benefit) than would be the case if a sealed reactor were employed and the concentration of monomer in solution in the reactor achieved its equilibrium value. Economically, this increase in recycle volume means greater expense than would otherwise be the case. See U.S. Pat. No. 3,706,719. Moreover, if the reaction temperature is increased above the critical temperature of monomer, the monomer mass transfer problem becomes more acute since the solubility of monomer will be lower, thereby reducing gas/liquid phase mixing.
In addition to monomer imbalance in the vapor space and mass transfer problems, evaporatively cooled reactors also lead to the associated problem of reactor fouling and polymer segment formation. More specifically, because various .alpha.-olefins possess different reactivities, they co-polymerize at different rates. Moreover, because more volatile .alpha.-olefin homopolymerizes much faster than it copolymerizes with less volatile .alpha.-olefin, the copolymerization of more volatile .alpha.-olefin with other .alpha.-olefins can result in polymers having large crystalline polymer segments randomly interspersed with occasional other .alpha.-olefin moieties.
These phenomena not only make it difficult to control the composition in the polymer, reduce the solubility of the polymer in the reaction mixture, and consequently lead to reactor fouling, but also more importantly, they limit the utility of the polymer in applications extremely sensitive to crystallinity such as to make dispersants for lubricating oil compositions.
The conventional solution to controlling polymer content, when using Ziegler-Natta catalysts, has been to regulate the concentrations of .alpha.-olefins in the reaction mixture. For example, to obtain a copolymer of propylene and higher .alpha.-olefin having approximately 50 mole percent of each monomer in the copolymer, it has been considered that a large excess of higher .alpha.-olefin, e.g., greater than 10:1 mole ratio, is necessary in the catalyst-containing solution in the reactor. In contrast, a copolymerization conducted in a solution containing about equal amounts of propylene and higher .alpha.-olefin, produces a copolymer so high in propylene content, that under ordinary Ziegler-Natta polymerization conditions, e.g., about -20.degree. to about 80.degree. C., it would not be soluble in the saturated hydrocarbon solvents used as the polymerization medium. However, when propylene and higher .alpha.-olefin, for example, are polymerized in a reactor having both liquid and vapor phases, the mole or weight ratio of higher .alpha.-olefin to propylene in the vapor phase is typically far less than the corresponding ratio in the liquid phase because of the greater volatility of propylene. For example, if the higher .alpha.-olefin:propylene mole ratio in the liquid phase is about 10:1, the mole ratio in the vapor phase above it may be only about 1:1 to about 3:1.
Uniformity of monomer incorporation, known as "compositional distribution", is also a function of the mass transfer into the reaction zone, i.e., uniform mixing of the co-monomers. However, as discussed above, in those reactor designs which employ recycle of the vapor phase, e.g., using a reflux condenser, the reflux condensate returning to the reactor will typically have sufficiently high more volatile monomer concentration such that reactor agitation of fresh and recycled monomer alone will not suffice to prevent insoluble polymers having randomly high more volatile monomer content from forming and clogging up the system. Consequently, it has been conventional in the art to attempt to introduce process steps for reducing the more volatile monomer content in the recycled condensate, e.g., by removing more volatile monomer from the condensate before introduction into the polymerization reactor. See U.S. Pat. Nos. 3,706,719 (col. 5, line 68 et seq.); 3,637,616; and 3,912,698. Such steps are costly and inefficient.
Separate and distinct from the need to control monomer ratio in the recycle stream are the mass transfer problems associated with employing pure feeds, particularly mixed pure feeds, even when supplied to reaction zones employing a solvent which dilutes the pure feed as it is introduced into the reactor. For example, the introduction of pure feeds into liquid reaction mixtures necessarily creates a higher concentration gradient of monomer at its point of introduction relative to the remainder of the reactor. Thus a finite amount of time will be required to achieve uniform mixing of the monomer into the reaction mixture. As long as this higher concentration gradient exists, there will be a propensity to form higher molecular weight polymer species relative to the molecular weight of polymer species formed at monomer equilibrium concentrations, since molecular weight is a function of monomer concentration. Broadened MWD and non-uniform compositional distribution are a result.
In view of the above, there has been a continuing need to develop more cost efficient processes for preparing olefin copolymers with metallocene catalyst systems; the present invention was developed in response to this need.